The technique of down-the-hole (DTH) percussive hammer drilling involves the supply of a pressurised fluid via a drill string to a drill bit located at the bottom of a bore hole. The fluid acts to both drive the hammer drilling action and to flush rearwardly dust and fines resultant from the cutting action, rearwardly through the bore hole so as to optimise forward cutting.
Typically, the drill assembly comprises a casing extending between a top sub and a drill bit. A piston is capable of shuttling axially between the top sub and the drill bit and is driven by the pressurised fluid so as to be configured to strike a rearward anvil end of the bit to provide the percussive action. A foot valve extends axially rearward from the drill bit to mate with the piston during its forwardmost stroke to control both the return stroke and provide exhaust of the pressurised fluid from the drill head which act to flush rearwardly the material cut from the bore face. Example DTH hammer drills are described in U.S. Pat. No. 4,278,135; U.S. Pat. No. 6,125,952, WO 97/00371; WO 2006/116646; WO 2008/051132 and WO 2013/104470.
The foot valve is repeatedly contacted by the reciprocating piston and is positioned at the region of contact between the piston and an anvil surface of the drill shank. Accordingly, the foot valve is subjected to mechanical and thermal stress and abrasion wear within the drill assembly that limits its operational lifetime. To replace the foot valve, it is necessary to extract the entire length of drill string loaded down the bore hole which is a time consuming exercise and is expensive due to lost drilling. US 2011/0232922 describes a variety of different foot valve embodiments in an attempt to maximise the service life of the valve to mitigate premature detachment of all or part of the valve during use. However, conventional foot valves and DTH drill assemblies are disadvantageous for a number of reasons. Typically, the foot valve is swaged or press-fitted into the drill bit shank which necessitates a mechanical or pneumatic/hydraulic press that is not typically available on-site. Additionally, and following use or damage, removal of conventional foot valves is difficult and time consuming adding to drilling downtime. For example, it is not uncommon for operators, on-site to continue installation and use of a foot valve that has been damaged during transportation or initial assembly as firstly it is difficult to remove the valve and secondly the time delay with returning the foot valve and assembly to the initial swaging press (commonly at a different location) is undesirable. Accordingly, what is required is a foot valve and/or drill assembly that addresses the above problems.